The present invention concerns the field of methods for managing access to a shared communication medium by a plurality of radio communication devices.
The invention is situated in the context of radio communication systems of the TDMA (Time Division Multiple Access) type.
The system is TDMA in that the time is divided into frames used for communication. Each frame is itself divided into elementary time intervals, called time slots, for communication. This communication is also multi-frequency, meaning the system can communicate on a plurality of frequencies. The transmission channel can therefore be seen as an arrangement with two dimensions, a time dimension and a frequency dimension, which defines elementary communication units (slots) corresponding to a time slot and frequency pair enabling communication.
Typically, although this is not always the case, the system is semi-duplex in that communication from base to handset, referred to as downlink communication, and communication from handset to base, referred to as uplink communication, are not simultaneous, but distributed over different time slots. Typically, the frame is divided into two parts, one part dedicated to downlink traffic and one part dedicated to uplink traffic.
FIG. 1 illustrates the structure of the frame in the example embodiment of the invention. The frame 1.1 is divided into a first part 1.2 dedicated to the downlink traffic sent by the base to the handsets, and a second part 1.3 dedicated to the uplink traffic from the handsets to the base. It is expressed along the time axis 1.4 and the frequency axis 1.5. It defines elementary communication units 1.6 defined by a given frequency and with a duration equivalent to an elementary time slot. The duration of the frame is, in the example embodiment, around 10 ms, divided into twice 12 elementary time slots. This frame is repeated periodically.
The communication system that is the context of the invention also has a broadcast channel. This broadcast channel is a communication medium shared by a plurality of communication terminals. Each terminal is allowed to send over this shared channel messages or data intended for all the terminals accessing the broadcast channel. This broadcast channel is implemented in the form of at least one elementary communication unit 1.7 in the frame. In some embodiments, the broadcast channel consists of a pair of corresponding elementary communication units 1.7 and 1.8 in each half-frame. One of the units is then used by the terminal for sending data, the other one is used for reception according to the scheme of the frame. The channel may also consist of a plurality of units if needed.
Since the broadcast channel is shared between the terminals, it may happen that several terminals decide to send over this channel at the same moment and that collisions occur preventing the reception of the data. It is therefore necessary to provide a method for managing collisions in this system.